1. Field of the Invention
The present invention relates to a preliminary process for forming a shallow trench isolation in a semiconductor device, more specifically, to a method for forming a shallow trench, which can avoid the occurrence of the striation phenomenon.
2. Description of the Prior Art
In the process for semiconductor devices such as DRAM, the formation of shallow trenches plays an important role.
As shown in FIG. 1, in the process of forming a shallow trench in a DRAM, an oxide layer 12 is formed on a substrate 10, a nitride layer 14 is formed on the oxide layer 12, and a BSG (borosilicate glass) 16 is formed on the nitride layer 14.
Subsequently, a photoresist layer 18 is formed, as shown in FIG. 2. In accordance with general mask etching process, through exposing, developing and imaging, the photoresist layer 18 defines the position to be etched. At the position defined by the photoresist layer 18, the BSG layer 16, nitride layer 14 and oxide layer 12 are etched to form an opening 20. Then the photoresist layer 18 is removed. However, due to the standing wave phenomenon of the light in the exposing step, the sidewall of the photoresist layer 18 is ribbed such that the sidewall of the etched opening is also ribbed, as shown in FIG. 3. Such a condition is referred to striation phenomenon.
In the structure of FIG. 3, the remaining BSG layer 16, nitride layer 14 and oxide layer 12 are used as a hard mask for the subsequent step of etching the substrate 10 to form a shallow trench. However, since the sidewall of the hard mask is ribbed, the sidewall of the etched shallow trench also presents the striation phenomenon. Accordingly, the profile of the shallow trench is degraded. As shown in FIG. 4, when the shallow trench, which presents striation phenomenon, is filled with dielectric material to form a shallow trench isolation (STI), the critical dimensions of the portions of different depths of the STI are not uniform, causing the isolating effects unstable.
Therefore, there is a need for a solution to overcome the problems stated above. The present invention satisfies such a need.